In the 1920s the German biochemist Otto Warburg described metabolic differences between cancerous and normal cells, where he noted that tumor cells rely upon a high rate of aerobic glycolysis rather than oxidative phosphorylation to produce energy for maintenance of cellular functions.1,2 Indeed, cancer cells have up to a 60-fold enhanced rate of glycolysis relative to normal cells, even with sufficient oxygen.1 This dependence upon glycolysis, and its consequences, is termed “the Warburg effect”.2 
Malignant cells are highly anabolic and require and require very high levels nutrients, ATP and building blocks to synthesize components needed for their growth and survival. Use of the glycolytic pathway provides ATP but also drives production of lactate, which is produced from pyruvate at the end of the glycolytic pathway. Massive lactate production by the tumor cell requires an efficient means for its consumption or elimination, to prevent intracellular acidification of the cancer cell.
Two mechanisms for handling excess lactate have been described. First, in some rare tumor types lactate is converted to pyruvate for entry into the TCA cycle. More commonly, lactate homeostasis is maintained via a family of twelve-membrane pass cell surface proteins known as the monocarboxylate transporters (MCTs; also known as the SLC16a transporter family). Fourteen MCTs are known, but only MCT1, MCT2, MCT3 and MCT4 transport small monocarboxylates such as lactate, pyruvate and ketone bodies (acetoacetate and β-hydroxybutyrate) across plasma membranes in a proton-linked exchange.3 Expression profiling studies have established that most aggressive tumor types express markedly elevated levels of MCT1, MCT4 or both.4 The chaperone protein CD147, which contains immunoglobulin-like domains, is required for MCT1 and MCT4 cell surface expression and is co-localized with the transporters. MCT1, MCT4 and CD147 are now high priority targets for cancer therapeutics.4 
The expression of MCT1 and MCT4 is regulated by two major oncogenic transcription factors, MYC and hypoxia inducible factor-1α (HIF-1α), respectively,4,5 that direct marked increases in the production of key proteins that support aerobic glycolysis, including amino acid transporters and enzymes involved in the catabolism of glutamine and glucose.6 Malignancies having MYC involvement and hypoxic tumors are generally resistant to current frontline therapies, with high rates of treatment failure, relapse and high patient mortality.7,8 Importantly, inhibition of MCT1 or MCT4 can kill tumor cells ex vivo and provoke tumor regression in vivo,4,9 and their potency is augmented by agents such as metformin that force a glycolytic phenotype upon the cancer cell.4 
Many weak MCT inhibitors (i.e., those effective at high micromolar levels) have been described, including α-cyano-4-hydroxycinnamate10,11 stilbene disulfonates,12 phloretin13 and related flavonoids.14 Coumarin-derived covalent MCT inhibitors have also recently been disclosed,15,16 as have pteridinones.17 
The most advanced MCT1 inhibitors are related pyrrolopyrimidine diones, pyrrolopyridazinones, and thienopyrimidine diones,18-23 including a compound that has advanced into clinical trials for treating some human malignancies.24,25 These compounds, and to our knowledge all MCT1 inhibitors yet described, are dual MCT1/MCT2 inhibitors. MCT2 has very high sequence homology with MCT1, yet it likely has a lesser role than MCT1 and MCT4 for monocarboxylate transport in human cancers based upon expression studies. However, MCT2 inhibition may play a role in potential off-target effects of current agents that could arise from blocking lactate transport in normal cells.
The first highly potent MCT inhibitor was initially identified via a cell-based assay seeking immunosuppressive agents that inhibit NFAT1-directed IL-2 transcription.26 MCT1 inhibition as its mechanism of action was described a full decade later.18 Several subsequently published analogs are also potent MCT1 inhibitors, with low nanomolar Ki values for MCT1 inhibition and low nanomolar EC50 values inn MTT assays for growth of MCT1-expressing tumors.
In many human tumors MCT1 and MCT4 are inversely expressed. Small molecule MCT1 inhibitors are now known to disable tumor cell metabolism, proliferation and survival, and impair tumorigenic potential in vivo in tumors highly expressing MCT1.4 MCT4 inhibitors are likely to be similarly effective for tumors highly expressing MCT4. Antitumor effects of MCT1 inhibitors are augmented by co-administration of the biguanide metformin, which is thought to further augment reliance by tumor cells upon aerobic glycolysis and thus increase the demand to MCT1-mediated efflux of lactate.4 
In addition to antitumor effects, inhibitors of MCT1 and/or MCT4 may have other important biological effects, such as immune suppression,18 anti-inflammatory,26 and antidiabetic effects.27-32 MCT1 is normally expressed at very low levels in pancreatic islets and in beta-cells in particular.27-28 This scenario explains the very slow uptake of lactate in these cells.29 A hallmark of exercise-induced hyperinsulinism (EIHI) is inappropriate insulin secretion following vigorous physical activity, which leads to hypoglycemia.30 In a 2012 study, Rutter and co-workers established that EIHI is associated with elevated expression of MCT1 in beta-cells and that transgenic mice engineered to overexpress MCT1 in part displayed many of the hallmarks of EIHI6.31 While the link between lactate and insulin secretion has been suggested since the late 1980s32 these more recent studies clarify the central role of MCT1 (and perhaps of the related lactate transporters MCT2 and MCT4).